The most destructive aspect of volcanoes are lahars and pyroclasic flows. Lahars are volcanic mudflows created when water (from rain or meltwater from glaciers) and ash mix. This deadly combination can have devestating results on the surrounding area. When lahars settle they can be metres thick and as hard as cement. Lahars can occur long after a volcanic eruption. Pyroclastic flows are avalanches containing hot volcanic gases, ash and volcanic bombs. On steep volcanoes pyroclastic flows can reach speeds of over 100 miles per hour. back to top Why do people live close to volcanoes? Volcanoes have a wide range of effects on humans. These can be problematic or beneficial. It is usually the destructive nature of volcanoes which is more widely documented. However, many people rely on volcanoes for their everyday survival. Today, many millions of people live close to volcanoes for this very reason. People live close to volcanoes because Geothermal energy can be harnessed by using the steam